The present invention pertains generally to methods and systems for accomplishing hydrothermal treatment for the purposes of either waste destruction, energy generation, or the production of chemicals. More specifically, the present invention pertains to methods and systems for the hydrothermal treatment of solids having organic constituents. The present invention is particularly, but not exclusively, useful as a method and system for volatilizing a portion of a material and subsequently treating the volatilized portion hydrothermally.
The present invention pertains to a process for converting materials at supercritical temperature and pressure conditions, or at supercritical temperatures and elevated, yet subcritical, pressures. Supercritical and subcritical are defined here with reference to the critical point of pure water, 705xc2x0 F. and 218 atm. For example, U.S. Pat. No. 4,338,199, which issued on Jul. 6, 1982 to Modell, discloses a hydrothermal process known as supercritical water oxidation (SCWO) because in some implementations oxidation in the aqueous/steam matrix occurs essentially entirely at conditions supercritical in temperature and pressure. The SCWO process has been shown to give rapid and complete oxidation of virtually any organic compound in a matter of seconds at 1000-1250xc2x0 F. and 250 atm.
Under SCWO conditions, carbon and hydrogen form the conventional combustion products CO2 and H2O, while chlorinated hydrocarbons (CHC""s) give rise to hydrochloric acid (HCl). If cations are available, they will react with the hydrochloric acid to form chloride salts. Alkali may be intentionally added to the reactor to avoid high, corrosive concentrations of hydrochloric acid in the reactor and especially in the cooldown equipment following the reactor. One advantage of the SCWO process is that the conversion of material can be accomplished without producing the environmentally harmful by-products that are produced when the same material is combusted in air. For example, the final product of sulfur oxidation in SCWO is sulfate anion, in contrast to normal combustion, wherein sulfur oxidation forms gaseous SO2. As in the case of chloride, alkali may be intentionally added to avoid high concentrations of sulfuric acid. Similarly, the SCWO product of phosphorus oxidation is phosphate anion.
A hydrothermal process related to SCWO known as supercritical temperature water oxidation (STWO) can provide similar oxidation effectiveness for certain feedstocks but at lower pressure. This process has been described in U.S. Pat. No. 5,106,513 issued Apr. 21, 1992 to Hong, and utilizes temperatures in the range of 1200xc2x0 F. and pressures between 25 and 218 atm. Like SCWO, the overall goal of the process may be waste destruction, energy generation, or production of chemicals. For convenience, the processes of SCWO and STWO will both be referred to herein as hydrothermal oxidation (HTO).
A key advantage of the hydrothermal processes described above is the cleanliness of the liquid and gaseous effluents. In particular, the gaseous emissions are far cleaner than those obtained by the conventional practice of incineration. EPA""s Maximum Achievable Control Technology (MACT) standards for hazardous waste incineration took effect on Sep. 30, 1999. Current operating facilities were given until Mar. 31, 2003 to comply with the regulations. New facilities are required to comply with the new regulations at start-up. Table 1 shows that HTO emissions meet the MACT standards with little or no post-treatment, while incinerators require extensive emissions cleanup.
A useful variation on the HTO process is that in which no oxidant, or a sub-stoichiometric amount of oxidant, is added to the reactor. In this case, rather than converting to CO2 and H2O, the organic material can reform into useful organic products. This process will be referred to as hydrothermal gasification (HTG), while HTO and HTG will be jointly referred to as hydrothermal processing (HTP).
A conventional limitation of HTP has been its application to bulk solids. The pressurized nature of the process typically requires that bulk solids be ground to a fine particle size to allow pumping into a high pressure reactor. Both grinding and pumping can require specialty equipment. In particular, a different device is generally required for different materials such as wood, plastic, or friable solids. Once the material has been ground, introduction into a pressurized reactor usually requires slurrying the material at a high concentration to minimize the size of the HTP reactor and associated process equipment. Thus, expensive, high pressure slurry pumps for viscous streams are typically required. For other solids such as metals, glass or ceramics, suitable size-reduction for introduction into an HTP reactor vessel is completely impractical.
A large amount of hazardous waste is generated each year that cannot be placed in a typical landfill unless it is pre-treated. Among this hazardous waste is a large amount of mixed waste consisting of non-hazardous solids that are contaminated with hazardous constituents. The hazardous constituents in these mixed-waste streams are generally suitable for direct feeding into a HTP reactor if they can be first separated from the solid portion of the waste stream. Once the hazardous constituent is extracted from the solid portion, the solid portion is generally considered non-hazardous and can be disposed of without further treatment in a conventional landfill.
Examples of such mixed-wastes include soils, inorganic adsorbents and other solids that are contaminated with hazardous organic or radioactive materials. Another such mixed waste consists of conventional and chemical munitions as well as munition dunnage. Protective suits, munition bodies and equipment contaminated with energetics, biological or chemical warfare agents is another mixed waste in which the solids portion could be disposed of conventionally if the hazardous contamination was removed and treated. Similarly, PCB contaminated transformers, pesticide contaminated bags and containers, and medical/biohazard waste such as contaminated needles and glass containers are all mixed wastes that could be disposed of efficiently by first separating the waste into hazardous and non-hazardous components.
Another category of waste that can pose difficulty for treatment by HTP is a concentrated acid, base or salt solution contaminated with an organic material. Treatment could be facilitated if the hazardous organic constituents could be separated for HTP while the residual inorganic solution could be handled by simpler means.
In the preceding examples, the organic to be treated may be a minor constituent or contaminant, or it may constitute a major portion of the feedstock.
In light of the above, it is an object of the present invention to provide methods suitable for the purposes of treating hazardous waste streams containing bulk solid materials and slurries that are difficult to size-reduce and pump to elevated pressure. It is another object of the present invention to provide methods for the removal and destruction of organic constituents from viscous materials such as bulk solids, sludges and slurries without having to pump the viscous material to high pressure. It is another object of the present invention to provide methods for the removal and destruction of organic constituents from acidic, alkaline, or salt-bearing waste streams or feedstocks. Yet another object of the present invention is to provide a method for chemically converting feedstocks containing solids using hydrothermal treatment which is robust, simple, and economical.
The present invention is directed to a system and method for treating feedstocks that include large solid objects, dissolved or undissolved solids, sludges or slurries that contain organics that may be volatilized. For the present invention, the feedstock is first fed into a desorption chamber to volatilize a portion of the feedstock and thereby separate the feedstock into a volatile portion and a residue portion. The feedstock can be continuously fed into the desorption chamber, or the feedstock can be introduced into the desorption chamber in batches. In the desorption chamber, the feedstock is heated to a temperature between approximately 300xc2x0 F. and approximately 1500xc2x0 F. and pressurized to a pressure of between approximately 20 atmospheres and approximately 200 atmospheres in an atmosphere that is overall net reducing.
In accordance with the present invention, steam, water or oxidants can be introduced into the desorption chamber to aid in the volatilization process. Specifically, these materials can be introduced to serve as reactants for localized partial oxidation and gasification reactions that assist in the overall volatilization of organic constituents. For the present invention, solids handling equipment can be installed in the desorption chamber to mix the steam with the feedstock and thereby increase the rate of volatilization. When continuous feed systems are employed, the solids handling equipment can also be utilized to transport the feedstock from the entrance to the exit of the desorption chamber. Examples of solids handling equipment that can be installed within the desorption chamber for use in conjunction with the present invention include augers, rotary kilns and drum or container breaching equipment.
Inside the desorption chamber, several mechanisms can be employed that operate alone or in combination to heat the feedstock. Sources of heat can include the use of conventional heating elements to heat the walls or solids handling equipment, the introduction of heated steam into the desorption chamber, and the heat generated from any exothermic reactions that occur in the desorption chamber.
From the desorption chamber, the volatile portion of the feedstock is transferred to a reactor for hydrothermal treatment, while the residue portion of the feedstock is removed from the desorption chamber for disposal. Preferably, a transfer pipe is used to establish fluid communication between the desorption chamber and the hydrothermal reactor to thereby transfer the volatile portion of the feedstock to the hydrothermal reactor. For the present invention, the volatile portion can be fed into a pipe reactor, downflow reactor or any other type of reactor suitable for hydrothermal treatment.
In the hydrothermal reactor, the volatile portion may be combined with an excess (20-100%) of oxidant and auxiliary fuel (if required). The reaction between the volatile portion, oxidant and auxiliary fuel is maintained at a temperature between approximately 1000xc2x0 F. and approximately 1800xc2x0 F. and a pressure of between approximately 20 atmospheres and approximately 200 atmospheres. The throughput of the reactor is controlled to cause the volatile portion of the feedstock to remain in the reactor for a sufficient residence time (10-60 seconds) to ensure complete oxidation of all organic species. After reaction, the effluent from the reactor can be further processed and then disposed.
Alternatively, addition of oxidant to the hydrothermal reactor may be reduced or eliminated, to allow organic reforming reactions to occur. Reactor temperature and pressure condition are again maintained at 1000-1800xc2x0 F. and 20-200 atmospheres.